How do we measure the speed of light?

The speed of light is said to be 299,792,458 m/s, without decimals. But, what if I tell you, that light may never travel at this speed?

We can’t measure the speed of light, the way we measure anything else. To measure the speed of any object , we need to know two things.

1. The distance between two points,

2. The time taken by that object to travel  between two points.

 When we divide the distance by time, we get the speed of that particular thing. 

DistanceFormula, speed

So is it possible to measure the speed of light the same way we measure the speed of different objects? Well, when you play with a laser, you don’t see the laser beam travelling to the spot you’ve pointed the laser to, it just appears to be a dot at that point at that very moment.

Laser Beam

Who were the first one to discover the speed of light?

Philosophers and ancient Greeks were the first to think about the speed of light. Empedocles, in the 5th century BCE claimed that light had a finite speed. However, Aristotle disagreed and claimed light to be instantaneous. Euclid and Ptolemy thought that light came from eyes which is what enabled vision, this would seem to imply that light had an infinite speed which allowed us to see really far objects like starts, the moment we open our eyes. 

Over a 1000 years later, Islamic philosopher Alhazen Ḥasan Ibn al-Haytham proposed that light actually travels from an object into our eyes and therefore has a finite speed. 600 years later, Kepler argued that the speed of light was infinite as there is nothing in space to slow down the speed. The debate continued for more than 2000 years. In 1629, Isaac Beeckman proposed an experiment which was inconclusive. Galileo came up with another experiment whos results were also inconclusive. 

Ole Romer

It was not until 1676 that someone would finally prove the speed of light was NOT infinite. He was a Danish astronomer Ole Romer. Romer was studying Io, the moon of Jupiter and it’s orbit around the planet which took place every 1.76 days. Knowing this information, Romer believed he could precisely predict and monitor Io, but unfortunately he couldn’t. He noticed sometimes Io seemed to be behind schedule and other times ahead of schedule, but what was causing this change?

Romer's prediction

What helped Romer prove the speed of light?

Well, after thinking about it for a while, Romer noticed that Io was behind schedule when Earth and Jupiter were further apart in orbit and ahead of schedule when both were close together. It was not that Io was changing its orbit, but just that light took less time to reach Earth when Jupiter was closer and took more time when it was further. This must mean that light has a finite speed.

Romer once predicted that on Nov 19 1676, Io’s eclipse would happen 10 mins later than the usual time expected. When the time arrived, everyone was astonished to see that eclipse just as Romer had predicted. With the help of Romer’s prediction, a Dutch scientist Christiaan Huygens was able to calculate the speed of light to be 220,000,000 m/s which is close to the figure we have today.

It was not until the 1970’s when lasers we finally introduced. Lasers made everything better and by using one the speed of light was determined as 299,792,456.2. A decimal? Umm.. well YES! Believe it or not in 1983 the definition of a meter was actually redefined so that the speed of light could be a whole number and that speed is 299,792,458 m/s. 


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